Burst alignment procedure

ABSTRACT

Burst alignment procedure between cordless terminals (T) and their base stations (BS), from which the cordless terminals (T) in question obtain the reference for their timebase. This burst alignment procedure is applicable in digital cordless communications systems with time division multiple access, to ensure the correct reception of the bursts received with different delays due to propagation effects. In this procedure, the cordless terminals (T) retransmit successively, up to a maximnum number of times, the first bursts to their base stations (BS), bringing them forward on each retransmission by a number of bits, depending on the inter-channel guard space, until an answer is received from their base stations (BS) once they are correctly synchronized with them.

TECHNICAL FIELD

This invention refers to a burst alignment procedure between a cordlessterminal and its base station. This procedure is of particularapplication in digital cordless communications systems with timedivision multiple access, to ensure the correct reception of burstsreceived with different delays due to propagation effects.

BACKGROUND OF THE INVENTION

Many communications systems, formed by a set of cordless terminals and anumber of base stations, were developed with reduced range (picocells)for indoor use, the separation between the cordless terminals and thebase stations being very small. Presently, the initial application ofthese systems is being reconsidered with a view to employing them inlarger spaces, where there is an increased distance between the basestations and the cordless terminals, for their use as an alternative tosystems based on cellular technology. In these communications systems,the cordless terminals obtain their timebase reference from the basestations, so that the transmitting instants of the base stationscoincide with those of reception in the cordless terminals, and viceversa.

Due to the configuration that the cordless systems were originally goingto have, the standards developed for this technology do not lay downguidelines for overcoming the problems resulting from propagation delaysdue to the distance between the base stations and the cordlessterminals, such as the problems of overlapping caused by the delay withwhich the messages are received with respect to the instant when theyshould have been received, resulting in the total or partial loss ofinformation channels. This is particularly critical in systems with timedivision multiple access TDMA, and with a high bit rate, where suchproblems can arise but which were practically of no consequence for thedistances between base stations and cordless terminals that existed inthe original configuration intended for these systems.

The European standard ETS 300 175-3 "Digital European CordlessTelecommunication (DECT), Common Interface Part 3: Medium Access ControlLayer", October 1992, developed by the RES (Radio Equipment and System)Technical Committee, in Annex D "Synchronization", page 186, references,for this DECT system, the three types of synchronization between thecordless terminals and base stations for correct transmission andreception of the bursts interchanged: burst synchronization, framesynchronization and multiframe synchronization.

This European standard does not incorporate any procedure forcompensating the delay due to propagation (which impedes the correctsynchronization between the base stations and the cordless terminals,and which causes overlapping problems), since this is not a criticalproblem in the applications for which the system was intended.Consequently this European standard does not establish solutions for theproblem that arises in base stations when the bursts coming from thedifferent cordless terminals associated with the base stations arereceived with different delays due to the variation in distancesexisting between these cordless terminals and the base stations, whichcan result in interference between the two equipments that frequentlyproduces the total or partial loss of information channels.

As a consequence of the foregoing, the technical problem to be resolvedlies in achieving a correct synchronization between cordless terminalsand their base stations when the distance between them is increased, sothat the bursts coming from these cordless terminals are correctlyreceived in the base stations, without the propagation delay due todistance preventing correct synchronization between the base station andthe cordless terminals, avoiding burst overlapping problems with totalor partial loss of information or channels.

SUMMARY OF THE INVENTION

This invention presents a burts alignment procedure between cordlessterminals and their base stations, when these cordless terminals arelocated at some distance from their base stations, this procedure beingof application in digital cordless systems.

The procedure consists in the cordless terminals bringing forward themoment at which they send their bursts with respect to the referenceobtained from the base stations, in the event of no answer having beenreceived in a determined period of time to a first burst transmitted bythese terminals to the base stations. This advance in time is repeatedup to a maximum number of bits, depending on the size of theinter-channel guard space used in the system, in order to attain correctsynchronization with their base stations and compensate the delayproduced in the propagation path of the signals transmitted by bothequipments, cordless terminals and base stations.

The advantage of using this procedure is that a greater range isachieved between the cordless terminals and their base stations, whichcan reach several kilometers, through compensating the propagation delaydue to distance, avoiding thereby the loss of channels and informationby overlapping at the point of reception. In this way it is possible toextend the range of application of the digital cordless systems,resulting in a competitive alternative to cellular systems.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller explanation of the invention can be found in the followingdescription of a preferred implementation based on the figures attached,in which:

FIG. 1 illustrates the system on which the burst alignment procedureobject of this invention has been implemented, and

FIG. 2 shows a flowchart of consecutive states in the system in theburst alignment procedure object of this invention, during the basestation locking phase.

BEST MODE FOR CARRYING OUT THE INVENTION

The diagram of FIG. 1 shows the cordless communications system in whichthe burst alignment procedure of this invention has been implemented inorder to ensure the correct reception in the base stations of the burstsreceived with different delays because of propagation effects. Itconcerns a radio access system for access by cordless subscribers to afixed network RF.

This system comprises a set of base stations BS that allow access fromthis system to the fixed network RF, and a set of cordless terminals T.The base stations BS are connected by wire to the fixed network RF. Thecordless terminals T are connected by radio with the base stations BS,these cordless terminals T being located at different distancesd1,d2,d3,d4 from their corresponding base stations BS. In thisimplementation, in order to extend the range of the cordless systems,directional antennas have been used in the base stations BS.

The technology employed in the system configuration is the European DECT(Digital European Cordless Telecommunications) standard, which makes useof TDMA frames of 11,520 bits and 10 ms duration, with 24 channels of416.6 ps, there being twelve channels for transmission and twelve forreception. Each of the 24 channels of 480 bits, numbered 0 to 479,includes a 56 bit guard space. Moreover the European standard providesfor a permissible shift in clocks that can be up to two bits.

The cordless terminals T have to synchronize their timebases with thoseof the base stations BS before they can obtain service, so that thetransmitting instants of the base stations coincide with those ofreceiving in the cordless terminals, and vice versa, and thecommunications between the two take place correctly.

This synchronization of the cordless terminals T is achieved through thereception at physical layer level of bursts sent from the base stationsBS. The possible states that the cordless terminals T can adopt, as afunction of their synchronization, are:

Non-locked State: defined as the state in which the cordless terminal Tis not synchronized at any level with any base station BS.

Pre-locked State: defined as the state in which the cordless terminal

T is receiving the frame and multiframe synchronizing references comingfrom the base station BS, as well as information on its identity, but isnot yet synchronized with the latter.

Locked State: defined as the state in which the cordless terminal T issynchronized at all levels with the base station BS, that is, it hasacquired burst, frame and multiframe synchronization.

The burst alignment procedure object of this invention is carried out bythe cordless terminal T in the state defined above as the pre-lockedstate.

FIG. 2 shows the flowchart of successive states in the system in theburst alignment procedure object of this invention, during the basestation locking phase.

Starting from an initial idle state 1, in which the cordless terminal Tis obtaining the reference for its timebase from the base station BS, itverifies, from the information it has received from the base station BS,that it has permission to communicate with the latter; at this point thesystem evolves to the start-to-lock state 2 in which the cordlessterminal T sends a burst to the base station BS. The system passes tothe answering burst waiting state 3, in which the cordless terminal Twaits a predetermined time to receive the answering burst to the firstburst it sent. If the answering burst coming from the base station BS isreceived by the cordless terminal T within the predetermined time, thecordless terminal T understands that it is correctly synchronized withthe base station BS and the system evolves to the correctsynchronization state 8 with the base station BS, the cordless terminalT then passing to the locked state.

In the event that the predetermined period for the answering burstwaiting time 3 expires before the cordless terminal T has received theanswering burst from the base station BS to the first burst it sent, thesystem evolves to the non-synchronized state 4, in which the cordlessterminal T is not synchronized correctly with the base station BSbecause the burst has not been received correctly in the base station BSdue to the propagation delay caused by the distance that separates thecordless terminal T from the base station BS.

The system evolves to the testing state 5, in which the cordlessterminal T checks if it can or cannot advance the instant of burstsending by one more bit, in order to compensate the delay in receptionof this first burst in the base station BS. The maximum number of bitsthat the cordless terminal T can advance the instant of burst sending is54, which corresponds to the inter-channel guard space (except for thetwo clock shift bits permitted by the standard), the cordless terminal Tadvancing the burst sending instant by one bit each time.

In the testing state 5, the cordless terminal T checks if it has stillnot reached the maximum number of advanceable bits; if the answer isaffirmative, the system evolves to the retransmitting state 6, in whichthe cordless terminal T then brings forward the instant of sending byone bit, and the system adopts once again the start-of-lock state 2.This process is repeated until the cordless terminal T manages toreceive the answering burst from the base station BS or until themaximum number of bits by which it can advance the instant of sendingits bursts is reached.

If the cordless terminal T attains this maximum number of bits withouthaving received the answering burst from the base station BS, theterminal then considers it is located at too great a distance andlocking on to the base station BS is impossible, at which point thesystem evolves to the release state 7.

I claim:
 1. A burst alignment procedure in digital cordlesscommunications systems with time division multiple access, between acordless terminal (T) and its base station (BS), from which thiscordless terminal (T) obtains its timebase reference; this procedurebeing characterised in that the cordless terminal (T) brings forward theinstant of sending its bursts with respect to the reference receivedfrom the base station (BS) in the event that no answer is received,within a preset period of time, from the base station (BS) to a firstburst transmitted by the cordless terminal (T) to its base station (BS).2. A burst alignment procedure according to claim 1, characterised inthat the cordless terminal (T) automatically repeats this advancement inthe time of transmitting its bursts by a number of bits each time untilit reaches correct synchronization with its base station (BS) in theevent that it continues not to receive an answer.
 3. A burst alignmentprocedure according to claim 2, characterised in that the total numberof bits by which the cordless terminal (T) brings forward the instant oftransmitting its bursts as well as the maximum number of times that thiscan be done, is limited by the size of the inter-channel guard space ofthe system.
 4. A burst alignment procedure according to claim 1,characterised in that the cordless terminals (T) are considered to besynchronized with the base stations (BS) when they receive an answer,within the preset period of time, to the burst sent to the base stations(BS).
 5. A cordless terminal (T) associated with a base station (BS) foruse in cordless digital communications systems, characterised in thatthe cordless terminal automatically and successively brings forward, upto a maximum number of bits, the moment of transmitting its bursts withrespect to the timebase reference obtained from the base station (BS) inthe event that no answer is received from this base station (BS) inresponse to the bursts sent by this cordless terminal (T) within apreset period of time.